Allow disabling the min and max version

Reviewed-by: Viktor Dukhovni <openssl-users@dukhovni.org>

ssl/ssl_conf.c

@@ -332,6 +332,7 @@ static int protocol_from_string(const char *value)
         int version;
     };
     static const struct protocol_versions versions[] = {
+        {"None", 0},
         {"SSLv3", SSL3_VERSION},
         {"TLSv1", TLS1_VERSION},
         {"TLSv1.1", TLS1_1_VERSION},

ssl/statem/statem_lib.c

@@ -834,6 +834,11 @@ int ssl_check_version_downgrade(SSL *s)
  */
 int ssl_set_version_bound(int method_version, int version, int *bound)
 {
+    if (version == 0) {
+        *bound = version;
+        return 1;
+    }
+
     /*-
      * Restrict TLS methods to TLS protocol versions.
      * Restrict DTLS methods to DTLS protocol versions.